A cathode for the electrolytic deposition of non-ferrous metals

Abstract: A cathode for the electrolytic deposition of non-ferrous metals, consisting of a plate (1) which, starting from one at least one of its vertical edges, is provided on both sides with coinciding zones (5) of dielectric material situated at the height of impingement of a number of penetrators which cause the upper edge of the deposited layers of zinc to detach. The plate (1) is provided with a slot (7), coinciding with said regions of dielectric material, which is occupied by a flat piece (5) of dielectric material, equal in thickness to the plate (1) and with surfaces which are flat and coplanar with those of said plate (1). The piece (5) and the slot (7) are provided with means of preventing the movement of said piece in any direction.

Description: The present invention relates to a cathode for the electrolytic deposition of non-ferrous metals such as zinc, copper etc., and more particularly to a flat cathode consisting of an aluminium plate, designed particularly for the production of zinc. BACKGROUND OF THE INVENTION In the cathodes of the type described above, the aluminium plate is usually terminated along its upper edge in a bar which is thicker and longer than said edge. With this arrangement the cathodes are situated in the electrolytic tanks with the aluminium plate in the vertical position, the bar acting as a head for supporting the cathode in the tank and for connecting the lifting apparatus which handles it. During the process of electrolysis the zinc is deposited progressively over the entire surface of the aluminium plate in the region which is submerged in the electrolytic tank. Generally, the vertical edges of the aluminium cathode plate are coated with a protective dielectric material which prevents the zinc from being deposited on the vertical edges. In this way the layers of zinc deposited on each surface of the aluminium plate are prevented from joining along the vertical edges of said plate. The layer of zinc deposited on the cathodes adheres to the surface of the aluminium plate. The devices used to detach this layer include two vertically moving knives terminating in converging horizontal lower edges, angular in section and of approximately the same length as the width of the cathodes. To use these devices for detaching the plates of zinc from the cathode one knife is situated on each side thereof, resting and pressing the edges of both knives against said surfaces, above the height reached by the deposits of zinc on the cathode. Then the knives are moved downwards such that when their edges meet the layer of zinc deposited on each side of the cathode they carry out its separation. This system has the drawback that knives erode the surfaces of the cathode over its entire height, making it unusable after a certain number of operations. Due to the high cost of the cathodes this system of detaching the zinc is therefore inadequate. An attempt to solve the above mentioned problems has been made by means of a system which includes two successive phases of action on the cathodes. During a first phase the upper edge of the deposited layers of zinc are detached by means of a horizontally acting lateral penetrator, and during a second phase the layers of zinc are pulled free entirely by means of two vertical knives. The lateral penetrator consists of two horizontal arms which have an acting end whose edge is in the form of a vertical angle wedge, said arms being able to move axially, one on each side of the cathode at the height of the upper edge of the deposited layer of zinc. Whilst these arms move the acting end thereof presses against the surface of the cathode and causes the erosion and deterioration thereof. In order to help the action of the lateral penetrator, the British patent No. 1.326.418 describes cathodes on which a pivoting piece made of a dielectric material is mounted on the upper part of one of the vertical edges, situated at the height of impingement and action of the penetrator, which corresponds to the height on the plate reached by the electrolyte in the tank. This piece is channelled and grips the cathode plate to which it is fixed by means of an axis which is perpendicular to said plate. The piece pivots about said axis between a lower position, in which it is coupled to and covers a certain region of the plate, as a prolongation of the protective profile of the vertical edges, preventing the zinc from being deposited thereon during the process of electrolysis, and an upper position in which said region is free and uncovered, enabling the arms of the lateral penetrator to move up to the surfaces of the cathode plate in this region and, as it continues to advance, meeting the edge of the zinc deposit in order to detach it. These cathodes have one fundamental drawback which derives from the need to rotate the pivoting piece from its lower position to its upper position each time that the zinc deposits are to be detached. As the cathode is introduced into the tank, the pivoting piece should occupy its lower position. The rotation of the pivoting piece and the conditions to which said piece is subjected cause the adjustment between the piece and the cathode, and fundamentally between the axis of rotation, to be lost and deteriorate in a relatively short period of time, the pivoting piece having to be raised by hand during the operation of detaching the zinc, with the corresponding increase in cost of the process in the form of labour and increase in the corrosion of the cathode sheet in this region. The German patent DE-A-3051150 describes a cathode whose plate has a fixed coating made of an insulating material starting from one of the vertical edges, in the region where the lateral penetrators impinge and rest. This coating extends over regions which coincide with the surfaces of the plate and over the section of the intermediate vertical edge. The external surfaces of the coating are flat and coplanar with those of the plate, for which both surfaces of said plate are provided with undercuttings, the shape and depth of which determine the shape and thickness of the coating of insulating material for each side of the plate. To prevent this coating from separating from the cathode plate, the region in which the undercuttings for fixing said coating are made may be provided with through holes via which the coating layers on each of the surfaces of the plate are joined. EP-A-470033 teaches a cathode for the production of zinc by electrolysis, consisting of an aluminium alloy plate, preferably of rectangular shape, with one of its shorter sides having a bar or head of a length greater than that side and fitted with rings or hooks for hanging, in that, on one of the sides perpendicular to this bar, the plate is provided with a zone of lesser thickness, shaped by recesses with matching contours, and preferably of equal depth, in the surface of the plate; this zone having a coating of material that is dielectric and acid-resistant material, with a flat external surface that is coplanar to the surface of the plate; said zone being located at and having, a height such that, its upper and lower edges are respectively above and below the maximum and minimum heights to which the plate is immersed in the electrolytic cell. The solutions detailed by these two publications have manufacturing problems due to the need to make the lowered regions on both surfaces of the cathode plate and the through holes between said regions and via which the coatings of both surfaces have to be joined. All of this implies a considerable increase in the cost of manufacture of the cathodes. Furthermore, due to the action of the lateral penetrators on the insulating coating, when they begin to act on the cathode they cause the premature breaking of the joins between the coatings of each side, causing them to separate from the plate. Furthermore, the thickness of the insulating coatings is relatively reduced and the action of the penetrators causes their premature deterioration, exposing part of the plate and thereby giving rise to irregular deposits of zinc which obstruct the action of the penetrators. This makes it necessary to carry out periodic revisions of the cathodes in order to replace the insulating coatings. DESCRIPTION OF THE INVENTION The object of the present invention is a cathode whose plate is provided with an insulating surface, in the region where the lateral penetrators impinge, starting from the corresponding vertical edge, the plate and insulating surface being constituted in such a way that the process of applying or assembling said surface can be carried out in a simple way so that the possible substitution of such a surface can be carried out in the same zinc producing plant, without having to resort to complicated devices or procedures. A further object of the invention is a surface of an electrically insulating material, with the characteristics described in the claims, which is very durable and which is formed on the surface of the cathode plate with practically no affect on the cost thereof. According to the present invention, the cathode for the electrolytic deposition of non-ferrous metals, consisting of a plate of aluminium alloy, or the like, preferably rectangular in shape, one of its shorter edges terminating in a bar or head which is greater in length, whilst starting from one or both of its longer edges it is provided on both sides of the plate with coinciding zones of dielectric material, situated at the height of the impingment of a number of horizontal penetrators which cause the upper edge of the deposited layers of zinc to detach, characterized in that starting from at least one of the edges which are perpendicular to said bar, the plate is provided with a slot situated close to said bar and coinciding with the regions of dielectric material, said slot having a height which is greater than the width of the penetrators and being occupied by a flat piece of acid resistant dielectric material, equal in thickness to the plate and with surfaces which are flat and coplanar with those of said plate and whose shape coincides with that of the slot, the piece and the slot being provided with means of preventing the movement of said piece in any direction, and consisting of a narrowing on both sides of the shape of the slot at its mouth, and a matching coupling between the opposing edges of said piece and those of the shape of the slot. The matching coupling is angular in section, with a convex shape along the slot and a concave shape along the opposing edge of the piece of dielectric material. This results in a cathode in which the insulating surfaces, situated in the regions where the lateral penetrators impinge, have considerable resistance since they are constituted by the insulating piece whose thickness is the same as that of the cathode. Furthermore, the formation of the insulating surfaces by means of the piece of dielectric material enables, during the deposition phase, an edge of sufficient thickness, in the form of a cord, to be obtained, adjacent to said insulating surfaces and which facilitates the initial action of the penetrators. In addition, the piece of dielectric material prevents the formation of undesirable deposits between the insulating material and the chamfers of the cathode plate. The slot or slots of the cathode plate are provided with means of preventing the piece of dielectric material from moving, both in its own plane and in the direction perpendicular thereto, thereby ensuring its immobility and positioning in the slot of the plate. BRIEF DESCRIPTION OF THE DRAWINGS These characteristics, as well as other characteristics of the present invention, are described below in greater detail with reference to the accompanying drawings which show by way of a non-limiting example one practical embodiment thereof. In the drawings: Figure 1 is a front elevation of a cathode constituted according to the present invention. Figure 2 is a profile view of the same cathode. Figure 3 corresponds to the detail A of figure 1 shown to a larger scale. Figure 4 is a view similar to that of figure 3 showing the cathode plate without the piece of insulating material. Figures 5 and 6 are sections taken along the lines V-V and VI-VI of figure 3 respectively. DESCRIPTION OF A PREFERRED EMBODIMENT The cathode shown in figures 1 and 2 includes a plate 1 rectangular in shape, made of aluminium, aluminium alloy, etc., and which has fixed on one of its transverse edges a bar 2 of greater length and width. This bar constitutes the head of the cathode and is used for supporting the cathode in the electrolytic tank and handling the cathode by means of lifting apparatus, for which it is provided with upper rings or hooks 3. The vertical edges of the plate are covered by means of a shape 4, made of a dielectric material, which prevents the zinc from being deposited on along these protected edges, such that the layers of zinc deposited on the surfaces of the plate 1 are not joined along these vertical edges. In this way, the zinc is deposited on the cathode on the free surfaces of the plate 1, forming two layers which are only joined together by their lower edge, around the lower edge of the plate 1. The plate 1 is also provided, starting from one of its vertical edges, with a region 5 of dielectric surfaces which are resistant to the acids used in electrolytic processes. The region 5 is situated close to the head bar 2, coinciding with the region of impingement and rest of a number of transverse penetrators, not shown in the drawings, for detaching the upper edge of the deposited layers of zinc. The upper edge 6 of the layers of zinc deposited on the plate 1, represented by the broken line in figures 1, 3 and 4, coincides with this region 5. Also shown is the axis 6a of the transverse penetrators. According to the present invention, the region 5 consists of a piece of plastic material which is acid resistant and equal in thickness to the plate 1. To mount this piece a slot 7 is made in the plate 1, starting from one of its vertical edges, said slot adopting any configuration and having a height which is greater than the width of the acting edge of the horizontal penetrators by means of which the upper portion of the layer of zinc deposited on the cathode is detached. The mouth of the slot 7 has a narrowing defined by the portions 8 and 9 which define opposing partial closures. The shape of the piece 5 coincides with that of the opening 7 such that it is prevented from separating or coming out of the plate by the portions 8 and 9. Furthermore, the shapes of the opposing edges of the piece 5 and the slot 7 are mutually matched. In the example shown in the drawings the edge 10 which defines the opening 7 is provided with chamfered edges, starting from both surfaces of the plate 1, defining a convex angular section, as shown in figures 5 and 6. The piece 5 is provided on its opposing edge with a channel which is concave and angular in section and which can be coupled to the angular edge 10. This system of matching or coupling between the two opposing edges of the piece 5 and the slot 7 prevents said piece from moving transversely relative to the plate 1. In this way the piece 5 of dielectric plastic material is deprived of all movement relative to the cathode plate 1. As has been indicated above, the piece 5 is equal in thickness to the plate 1, such that the major surfaces of said plate are coplanar with the surfaces of the plate 1, as can be seen in figures 5 and 6. With the characteristics described, regions 5 of dielectric material are obtained starting from the vertical edge of the plate 1 on which the horizontal penetrators impinge, said regions preventing the deposition of zinc. The penetrators slide easily over the surfaces of the piece 5 until they reach the layers of zinc which are deposited on the surfaces of the plate 1, causing them to separate from said plate, this operation being helped by the formation of cords whose edges are adjacent to the piece 5. The piece 5 may be moulded directly onto the shape of the opening 7 or be formed by two halves obtained according to the mid-plane parallel to the major surfaces of said piece. In the latter case, each of the two halves is joined on one side to the shape of the opening 7 and they are joined together by any system, for example by means of an acid resistant adhesive, thermo-welding, etc. The plate 1 may be provided with a slot 7 starting from each of its vertical edges, each of the two slots being occupied by pieces 5.

Claims: A cathode for the electrolytic deposition of non-ferrous metals, consisting of a plate (1) of aluminium alloy, or the like, preferably rectangular in shape, one of its shorter edges terminating in a bar or head (2) which is greater in length, whilst starting from one or both of its longer edges it is provided on both sides of the plate with coinciding zones (5) of dielectric material, situated at the height of the impingment of a number of horizontal penetrators which cause the upper edge of the deposited layers of zinc to detach, characterized in that starting from at least one of the edges which are perpendicular to said bar (2), the plate (1) is provided with a slot (7) situated close to said bar (2) and coinciding with the regions of dielectric material (5), said slot having a height which is greater than the width of the penetrators and being occupied by a flat piece (5) of acid resistant dielectric material, equal in thickness to the plate (1) and with surfaces which are flat and coplanar with those of said plate (1) and whose shape coincides with that of the slot (7), the piece (5) and the slot (7) being provided with means of preventing the movement of said piece (5) in any direction, and consisting of a narrowing on both sides of the shape of the slot (7) at its mouth, and a matching coupling (10,11) between the opposing edges of said piece and those of the shape of the slot. A cathode according to claim 1, characterized in that the matching coupling is angular in section, with a convex shape (10) along the slot and a concave shape (11) along the opposing edge of the piece of dielectric material.

Inventors: ALVAREZ TAMARGO FRANCISCO, LEFEVRE YVES, MARTINEZ VALDES JOSE MARIA, SITGES MENENDEZ FERNANDO, SITGES MENENDEZ FRANCISCO JAVI, ALVAREZ TAMARGO, FRANCISCO, LEFEVRE, YVES, MARTINEZ, VALDES, JOSE MARIA, SITGES MENENDEZ, FERNANDO, SITGES MENENDEZ, FRANCISCO JAVIER, Martinez, Valdes, José Maria, SITGES MENENDEZ, FRANCISCO JAVIER, LAS TEJAS
Applicants: ASTURIANA DE ZINC SA, ASTURIANA DE ZINC, S.A.
Application year: 1994
Main classification: C25C7
IPC codes: C25C7, C25C1